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    Subject[RFC PATCH v7 00/16] Add support for eXclusive Page Frame Ownership
    Date
    I am continuing to build on the work Juerg, Tycho and Julian have done
    on XPFO. After the last round of updates, we were seeing very
    significant performance penalties when stale TLB entries were flushed
    actively after an XPFO TLB update. Benchmark for measuring performance
    is kernel build using parallel make. To get full protection from
    ret2dir attackes, we must flush stale TLB entries. Performance
    penalty from flushing stale TLB entries goes up as the number of
    cores goes up. On a desktop class machine with only 4 cores,
    enabling TLB flush for stale entries causes system time for "make
    -j4" to go up by a factor of 2.614x but on a larger machine with 96
    cores, system time with "make -j60" goes up by a factor of 26.366x!
    I have been working on reducing this performance penalty.

    I implemented a solution to reduce performance penalty and
    that has had large impact. When XPFO code flushes stale TLB entries,
    it does so for all CPUs on the system which may include CPUs that
    may not have any matching TLB entries or may never be scheduled to
    run the userspace task causing TLB flush. Problem is made worse by
    the fact that if number of entries being flushed exceeds
    tlb_single_page_flush_ceiling, it results in a full TLB flush on
    every CPU. A rogue process can launch a ret2dir attack only from a
    CPU that has dual mapping for its pages in physmap in its TLB. We
    can hence defer TLB flush on a CPU until a process that would have
    caused a TLB flush is scheduled on that CPU. I have added a cpumask
    to task_struct which is then used to post pending TLB flush on CPUs
    other than the one a process is running on. This cpumask is checked
    when a process migrates to a new CPU and TLB is flushed at that
    time. I measured system time for parallel make with unmodified 4.20
    kernel, 4.20 with XPFO patches before this optimization and then
    again after applying this optimization. Here are the results:

    Hardware: 96-core Intel Xeon Platinum 8160 CPU @ 2.10GHz, 768 GB RAM
    make -j60 all

    4.20 915.183s
    4.20+XPFO 24129.354s 26.366x
    4.20+XPFO+Deferred flush 1216.987s 1.330xx


    Hardware: 4-core Intel Core i5-3550 CPU @ 3.30GHz, 8G RAM
    make -j4 all

    4.20 607.671s
    4.20+XPFO 1588.646s 2.614x
    4.20+XPFO+Deferred flush 794.473s 1.307xx

    30+% overhead is still very high and there is room for improvement.
    Dave Hansen had suggested batch updating TLB entries and Tycho had
    created an initial implementation but I have not been able to get
    that to work correctly. I am still working on it and I suspect we
    will see a noticeable improvement in performance with that. In the
    code I added, I post a pending full TLB flush to all other CPUs even
    when number of TLB entries being flushed on current CPU does not
    exceed tlb_single_page_flush_ceiling. There has to be a better way
    to do this. I just haven't found an efficient way to implemented
    delayed limited TLB flush on other CPUs.

    I am not entirely sure if switch_mm_irqs_off() is indeed the right
    place to perform the pending TLB flush for a CPU. Any feedback on
    that will be very helpful. Delaying full TLB flushes on other CPUs
    seems to help tremendously, so if there is a better way to implement
    the same thing than what I have done in patch 16, I am open to
    ideas.

    Performance with this patch set is good enough to use these as
    starting point for further refinement before we merge it into main
    kernel, hence RFC.

    Since not flushing stale TLB entries creates a false sense of
    security, I would recommend making TLB flush mandatory and eliminate
    the "xpfotlbflush" kernel parameter (patch "mm, x86: omit TLB
    flushing by default for XPFO page table modifications").

    What remains to be done beyond this patch series:

    1. Performance improvements
    2. Remove xpfotlbflush parameter
    3. Re-evaluate the patch "arm64/mm: Add support for XPFO to swiotlb"
    from Juerg. I dropped it for now since swiotlb code for ARM has
    changed a lot in 4.20.
    4. Extend the patch "xpfo, mm: Defer TLB flushes for non-current
    CPUs" to other architectures besides x86.


    ---------------------------------------------------------

    Juerg Haefliger (5):
    mm, x86: Add support for eXclusive Page Frame Ownership (XPFO)
    swiotlb: Map the buffer if it was unmapped by XPFO
    arm64/mm: Add support for XPFO
    arm64/mm, xpfo: temporarily map dcache regions
    lkdtm: Add test for XPFO

    Julian Stecklina (4):
    mm, x86: omit TLB flushing by default for XPFO page table
    modifications
    xpfo, mm: remove dependency on CONFIG_PAGE_EXTENSION
    xpfo, mm: optimize spinlock usage in xpfo_kunmap
    EXPERIMENTAL: xpfo, mm: optimize spin lock usage in xpfo_kmap

    Khalid Aziz (2):
    xpfo, mm: Fix hang when booting with "xpfotlbflush"
    xpfo, mm: Defer TLB flushes for non-current CPUs (x86 only)

    Tycho Andersen (5):
    mm: add MAP_HUGETLB support to vm_mmap
    x86: always set IF before oopsing from page fault
    xpfo: add primitives for mapping underlying memory
    arm64/mm: disable section/contiguous mappings if XPFO is enabled
    mm: add a user_virt_to_phys symbol

    .../admin-guide/kernel-parameters.txt | 2 +
    arch/arm64/Kconfig | 1 +
    arch/arm64/mm/Makefile | 2 +
    arch/arm64/mm/flush.c | 7 +
    arch/arm64/mm/mmu.c | 2 +-
    arch/arm64/mm/xpfo.c | 58 ++++
    arch/x86/Kconfig | 1 +
    arch/x86/include/asm/pgtable.h | 26 ++
    arch/x86/include/asm/tlbflush.h | 1 +
    arch/x86/mm/Makefile | 2 +
    arch/x86/mm/fault.c | 10 +
    arch/x86/mm/pageattr.c | 23 +-
    arch/x86/mm/tlb.c | 27 ++
    arch/x86/mm/xpfo.c | 171 ++++++++++++
    drivers/misc/lkdtm/Makefile | 1 +
    drivers/misc/lkdtm/core.c | 3 +
    drivers/misc/lkdtm/lkdtm.h | 5 +
    drivers/misc/lkdtm/xpfo.c | 194 ++++++++++++++
    include/linux/highmem.h | 15 +-
    include/linux/mm.h | 2 +
    include/linux/mm_types.h | 8 +
    include/linux/page-flags.h | 13 +
    include/linux/sched.h | 9 +
    include/linux/xpfo.h | 90 +++++++
    include/trace/events/mmflags.h | 10 +-
    kernel/dma/swiotlb.c | 3 +-
    mm/Makefile | 1 +
    mm/mmap.c | 19 +-
    mm/page_alloc.c | 3 +
    mm/util.c | 32 +++
    mm/xpfo.c | 247 ++++++++++++++++++
    security/Kconfig | 29 ++
    32 files changed, 974 insertions(+), 43 deletions(-)
    create mode 100644 arch/arm64/mm/xpfo.c
    create mode 100644 arch/x86/mm/xpfo.c
    create mode 100644 drivers/misc/lkdtm/xpfo.c
    create mode 100644 include/linux/xpfo.h
    create mode 100644 mm/xpfo.c

    --
    2.17.1

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